1. Field of the Invention
The present invention relates to a braking system for an unwinder of rolls of web material, and more particularly, to a braking system which maintains uniform tension on the web material downstream of the unwinding device as a roll of web material is unwound.
2. Prior Art
In unwinding devices such as those employed in the paper industry for unwinding large rolls of paper web it is generally necessary to apply some degree of braking force to the roll of web as it is being unwound in order to maintain a uniform tension on the web for processing through various types of paper processing machinery downstream of the unwinding apparatus. Some of the rolls of paper which are so processed are fairly large in diameter, for example sixty inches. This size of roll possesses fairly large inertia when it is rotated at the high rotational speeds necessary to achieve the high line speeds for processing of web material in modern equipment.
Since there are numerous occasions upon which the speed of the roll being unwound is either increased or decreased the braking forces applied to the roll must be adjustable in order to compensate for the inertia of the roll so as to maintain uniform tension in the web and prevent breakage of the web or reductions in tension. As the roll diameter decreases during the unwinding process the inertia of the roll likewise decreases and the braking forces required to maintain uniform tension should also be modified accordingly.
In less sophisticated prior art systems the braking force is adjusted manually by the operator maintaining vigilence over a tension measuring device which constantly monitors the tension on the web downstream from the unwinder and indicates to the operator whether or not the tension is as desired. Such a system obviously has several drawbacks particularly since the reaction time of the operator may not be sufficiently rapid to prevent breakage of the web should the tension increase rapidly for any reason. Also, it requires the operator to maintain constant vigilence over the tension and make multiple adjustments in the braking force as the roll diameter of the roll of web material being unwound decreases.
A more suitable prior art system utilizes a tension measuring device which provides continuous feedback to the braking system for automatic adjustment of the braking force when the tension in the web changes from the desired level. Such systems utilize the feedback signal to control the tension throughout the unwinding of the roll as well as for correcting for tension upsets which occur in the system. Since such systems do not take into account the roll diameter they must constantly increase or decrease the braking force applied to the roll until the proper tension level is reached.
The problem with this type of system is that it does not take into account changes in roll diameter during application of the braking force. Thus, a predetermined average braking force is usually programmed into the system and this braking force is too large to be applied to a roll which is almost completely unwound and too small to be applied to a new roll. With such systems, particularly when a roll has been substantially decreased from the nominal diameter used to establish the braking force, where almost instantaneous tension changes occur a rapid change in braking force would occur which could cause either braking of the web if the braking force is greatly increased or loss in tension due to a rapid releaving of the braking force.
In an attempt to solve the difficulties with the direct tension measuring feedback circuit, a system was developed which calculates the roll diameter continuously and applies proportional braking force to the roll to maintain a predetermined tension in the web. This is accomplished by measuring the line speed and the rpm of the roll of web material and then dividing the line speed by the rpm to determine the radius or diameter of the roll at that instant. The braking force is then proportioned for the given roll diameter by introducing a constant multiplier factor established by the operator for a desired web tension. The arbitrary constant is introduced by the operator into the braking force system by manual settings and is intended to compensate for the inertia of the roll. However, as the roll diameter decreases the actual inertia of the roll will change and thus the constant will not be accurate for the entire range of roll diameters as the roll is unwinding. Thus, with this system some variation in tension will occur due to the difference between the actual inertia of the roll and the calculated braking force based on the arbitrary constant intended to represent the roll inertia.